Do Your Homework

Louie Bauman

With the advent of four stroke motors offering greatly improved fuel economy, many anglers are looking that direction for a little relief at the pump. But what they are not looking at could cost them in the long run. Lower unit gear ratios play a significant part in the efficiency of your fishing rig and that is probably the most overlooked aspect of motor purchase due to the fact you have no options when it comes to manufacturer settings.

If you understand how gear ratios work in cars, this should be fairly easy to comprehend. If you are like me and most of it is a foreign language, I'll do my very best to put this as simply as possible and in terms even I understand.

A motor turns a certain RPM (revolution per minute). As the motor spins, the drive shaft is connected to a set of gears in the lower unit and those gears drive the propeller shaft. Now, the output shaft does not turn the same rpm's as the motor because of the planned difference in gear ratio. Due to this ratio, (which the manufacturer has set for that particular lower unit), the output shaft spins slower than the engine. That rotation speed can be figured by this simple formula: Engine RPM divided by gear ratio.

Knowing this particular information can be beneficial for a few reasons. Most important is: The slower a propeller spins; the more pitch you will need to maintain speed.

Now this is where being a math geek comes in handy. Using the first formula to determine prop shaft speed, you can now find out what the theoretical speed of your boat should be. Theoretical speed is the speed the boat would do with 0% slip. Take your engine RPM - divided by gear ratio now multiply that number by the pitch setting of the propeller - and divide that by 1056 (translates feet per second into miles per hour).

As an example, at full throttle your boat's outboard turns 5500 RPM. You are running a standard lower unit with a gear ratio of 1.86:1 and you are turning a 17 pitch propeller. Theoretically, you should be doing 47.6 mph.

Now go to one of today's new four strokes with a 2.71:1 gear ratio and spin the motor 6000 RPM's and go to a 21 pitch propeller. Before you do the math, guess which one should go faster.

If you did the math correctly, you find out that even though you are spinning a higher pitch propeller at a higher engine RPM, because of the gear ratio, it actually spins the prop slower and your theoretical speed is only 44 MPH.

This is what many of today's fishermen are running into and it is a propeller man's worst nightmare. The secret is to find out how to spin a higher pitched propeller faster and not overload the motor. That is the aspect that many boaters overlook. Adding pitch to a propeller gives you more speed as long as you can spin it within your recommended RPM range set by the manufacturer. But adding pitch adds more load to the motor, which in turn causes a reduction of engine RPM's and that could seriously hurt a motor.

Let's look at the chief reason the new motors are going to these lower gear ratios. (Lower because as the drive ratio number goes up, the fewer revolutions the output shaft will turn). In lower gear ratios, you are able to turn higher pitch propellers because the propeller is turning much slower and the load is not near as great- so you are able to take a bigger bite. So, spinning a propeller at a slower rate but taking a bigger bite is good for fuel economy.

Problem for shallow water tunnel boats or any boat that has lots of drag the harder the boat is to push, the more power the motor must deliver. And as you approach the power limit of the engine, to avoid pulling the engine's RPM into an undesirably low range, you find it necessary to reduce pitch. And now you're right back where you started.

Thus far we have limited our discussion to engine RPM, lower unit drive ratios, and propeller pitch. Luckily, we have several other formulas and prop performance characteristics at our disposal to get the job done. Being in possession of these data and a few other tuning tricks are what makes or breaks a good running boat.

I am in no way trying to steer you away from a new four stroke motor. What I am trying to do is provide some insight into the reason why four strokes could be causing some top speed performance issues. Not all shallow water boats are designed identically, therefore not all suffer from this problem, but some do. Knowing how to do the math and which questions to ask before you buy could make buying and setting up a new outboard a little less painful. Whichever you decide and buy, do the math to maximize your output.